Optical element with a self-assembled monolayer, lithographic projection apparatus including such an optical element, and device manufacturing method

ABSTRACT

A lithographic apparatus contains an optical element, the surface of the optical element being modified to reduce the effects of reflectivity reduction by molecular contamination. The surface includes a self assembled monolayer.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.10/459,706, filed Jun. 12, 2003, which claimed priority to EuropeanApplication 02254176.7, filed Jun. 14, 2002, the contents of bothapplications being incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a lithographic projection apparatusincluding an optical element with a self-assembled monolayer, an opticalelement with a self-assembled monolayer, and a device manufacturingmethod.

2. Description of the Related Art

The term “patterning device” as here employed should be broadlyinterpreted as referring to device that can be used to endow an incomingradiation beam with a patterned cross-section, corresponding to apattern that is to be created in a target portion of the substrate. Theterm “light valve” can also be used in this context. Generally, thepattern will correspond to a particular functional layer in a devicebeing created in the target portion, such as an integrated circuit orother device (see below). An example of such a patterning device is amask. The concept of a mask is well known in lithography, and itincludes mask types such as binary, alternating phase-shift, andattenuated phase-shift, as well as various hybrid mask types. Placementof such a mask in the radiation beam causes selective transmission (inthe case of a transmissive mask) or reflection (in the case of areflective mask) of the radiation impinging on the mask, according tothe pattern on the mask. In the case of a mask, the support willgenerally be a mask table, which ensures that the mask can be held at adesired position in the incoming radiation beam, and that it can bemoved relative to the beam if so desired.

Another example of a patterning device is a programmable mirror array.One example of such an array is a matrix-addressable surface having aviscoelastic control layer and a reflective surface. The basic principlebehind such an apparatus is that, for example, addressed areas of thereflective surface reflect incident light as diffracted light, whereasunaddressed areas reflect incident light as undiffracted light. Using anappropriate filter, the undiffracted light can be filtered out of thereflected beam, leaving only the diffracted light behind. In thismanner, the beam becomes patterned according to the addressing patternof the matrix-addressable surface. An alternative embodiment of aprogrammable mirror array employs a matrix arrangement of tiny mirrors,each of which can be individually tilted about an axis by applying asuitable localized electric field, or by employing piezoelectricactuators. Once again, the mirrors are matrix-addressable, such thataddressed mirrors will reflect an incoming radiation beam in a differentdirection to unaddressed mirrors. In this manner, the reflected beam ispatterned according to the addressing pattern of the matrix-addressablemirrors. The required matrix addressing can be performed using suitableelectronics. In both of the situations described hereabove, thepatterning device can include one or more programmable mirror arrays.More information on mirror arrays as here referred to can be seen, forexample, from U.S. Pat. Nos. 5,296,891 and 5,523,193, and PCTpublications WO 98/38597 and WO 98/33096. In the case of a programmablemirror array, the support may be embodied as a frame or table, forexample, which may be fixed or movable as required.

Another example of a patterning device is a programmable LCD array. Anexample of such a construction is given in U.S. Pat. No. 5,229,872. Asabove, the support in this case may be embodied as a frame or table, forexample, which may be fixed or movable as required.

For purposes of simplicity, the rest of this text may, at certainlocations, specifically direct itself to examples involving a mask andmask table. However, the general principles discussed in such instancesshould be seen in the broader context of the patterning device ashereabove set forth.

Lithographic projection apparatus can be used, for example, in themanufacture of integrated circuits (IC's). In such a case, thepatterning device may generate a circuit pattern corresponding to anindividual layer of the IC, and this pattern can be imaged onto a targetportion (e.g. including one or more dies) on a substrate (silicon wafer)that has been coated with a layer of radiation-sensitive material(resist). In general, a single wafer will contain a whole network ofadjacent target portions that are successively irradiated via theprojection system, one at a time. In current apparatus, employingpatterning by a mask on a mask table, a distinction can be made betweentwo different types of machine. In one type of lithographic projectionapparatus, each target portion is irradiated by exposing the entire maskpattern onto the target portion at once. Such an apparatus is commonlyreferred to as a wafer stepper. In an alternative apparatus, commonlyreferred to as a step-and-scan apparatus, each target portion isirradiated by progressively scanning the mask pattern under the beam ofradiation in a given reference direction (the “scanning” direction)while synchronously scanning the substrate table parallel oranti-parallel to this direction. Since, in general, the projectionsystem will have a magnification factor M (generally <1), the speed V atwhich the substrate table is scanned will be a factor M times that atwhich the mask table is scanned. More information with regard tolithographic devices as here described can be seen, for example, fromU.S. Pat. No. 6,046,792.

In a known manufacturing process using a lithographic projectionapparatus, a pattern (e.g. in a mask) is imaged onto a substrate that isat least partially covered by a layer of radiation-sensitive material(resist). Prior to this imaging, the substrate may undergo variousprocedures, such as priming, resist coating and a soft bake. Afterexposure, the substrate may be subjected to other procedures, such as apost-exposure bake (PEB), development, a hard bake andmeasurement/inspection of the imaged features. This array of proceduresis used as a basis to pattern an individual layer of a device, e.g. anIC. Such a patterned layer may then undergo various processes such asetching, ion-implantation (doping), metallization, oxidation,chemo-mechanical polishing, etc., all intended to finish off anindividual layer. If several layers are required, then the wholeprocedure, or a variant thereof, will have to be repeated for each newlayer. It is important to ensure that the overlay (juxtaposition) of thevarious stacked layers is as accurate as possible. For this purpose, asmall reference mark is provided at one or more positions on the wafer,thus defining the origin of a coordinate system on the wafer. Usingoptical and electronic devices in combination with the substrate holderpositioning device (referred to hereinafter as “alignment system”), thismark can then be relocated each time a new layer has to be juxtaposed onan existing layer, and can be used as an alignment reference.Eventually, an array of devices will be present on the substrate(wafer). These devices are then separated from one another by atechnique such as dicing or sawing, whence the individual devices can bemounted on a carrier, connected to pins, etc. Further informationregarding such processes can be obtained, for example, from the book“Microchip Fabrication: A Practical Guide to Semiconductor Processing”,Third Edition, by Peter van Zant, McGraw Hill Publishing Co., 1997, ISBN0-07-067250-4.

For the sake of simplicity, the projection system may hereinafter bereferred to as the “lens.” However, this term should be broadlyinterpreted as encompassing various types of projection system,including refractive optics, reflective optics, and catadioptricsystems, for example. The radiation system may also include componentsoperating according to any of these design types for directing, shapingor controlling the beam of radiation, and such components may also bereferred to below, collectively or singularly, as a “lens”. Further, thelithographic apparatus may be of a type having two or more substratetables (and/or two or more mask tables). In such “multiple stage”devices the additional tables may be used in parallel or preparatorysteps may be carried out on one or more tables while one or more othertables are being used for exposures. Dual stage lithographic apparatusare described, for example, in U.S. Pat. Nos. 5,969,441 and 6,262,796.

Because no material with suitable optical properties for makingrefractive optical elements for extreme ultraviolet (EUV) radiation isknown, lithographic apparatus using such radiation must use reflectiveoptics, made of grazing incidence mirrors or multilayer stacks.Multilayer stacks have reflectivities of a theoretical maximum of onlyabout 70%. In view of this lower reflectivity, it is important to ensurethat any drop in reflectivity due to molecular contamination isminimized. In spite of the high vacuum conditions which are imposedduring use, molecular contaminants may be present within an EUVlithography apparatus. The reflectivity of the optical elements cantherefore be reduced through oxidation of the top, e.g. silicon, layerof the mirror and also carbon growth on the surface of the mirror.Oxidation of the surface of the mirror under EUV radiation may be causedby the presence of water, while carbon growth, on the other hand, mayoccur due to the presence of hydrocarbons in the system.

SUMMARY OF THE INVENTION

It is an aspect of the present invention to provide optical elements foruse with EUV radiation, in particular for use in an EUV lithographyapparatus, wherein the loss of reflectivity caused by molecularcontamination is reduced or alleviated.

This and other aspects are achieved according to the invention in alithographic apparatus including a radiation system configured toprovide a beam of EUV radiation; a support configured to support apatterning device, the patterning device configured to pattern the beamof radiation according to a desired pattern; a substrate tableconfigured to hold a substrate; and a projection system configured toproject the patterned beam onto a target portion of the substrate,wherein the radiation system and/or the projection system contains anoptical element including a substrate; a Mo/Si multilayer stack on thesubstrate; and a hydrophobic self-assembled monolayer on the multilayerstack.

The present invention also relates to an optical element which has apeak of reflectivity in the wavelength range 5 to 50 nm, wherein theoptical element has, on its surface, a hydrophobic self-assembledmonolayer. The wavelength range is typically below about 20 nm and forexample below about 15 nm. Examples of wavelengths of interest are 13.5nm and in the region of 11 nm. In this context, reflectivity istypically greater than 40%, for example greater than 50% and for examplegreater than 60%.

The optical element may be a beam modifying element such as a reflector,e.g. a multilayer near-normal incidence mirror, especially a Mo/Simultilayer mirror, or a grazing incidence mirror, included in at leastone of the illumination system and the projection system; an integrator,such as a scattering plate; the mask, especially if a multilayer mask;or any other optical element involved in directing, focussing, shaping,controlling, etc. the beam of radiation. The optical element may also bea sensor such as an image sensor or a spot sensor.

The term “self-assembled monolayer” refers to a film composed ofmolecules that assemble themselves on a surface, by any mechanism,directly or via an intermediary and includes Langmuir-Blodgett films.

The self-assembled monolayer may be formed by the reaction of amodifying agent with the surface of the optical element. The modifyingagent is any type of amphiphilic species, provided that the reactionbetween the modifying agent and the surface of the optical elementresults in the formation of a self assembled monolayer. Generally, theamphiphilic species will possess an alkyl chain and be functionalised toreact with the surface. The nature of the functionality will depend uponthe surface of optical element. Generally, the surface will be asilica-type surface and the preferred amphiphilic species will be afunctionalised alkyl, for example an alkylsilane. The surface of theoptical element will thus be covered with an alkylsilane-based selfassembled monolayer. However, depending upon the surface of the opticalelement, other amphiphilic species may be used, for example carboxylicacids can be used on an alumina-type surface.

The alkylsilane-based self-assembled monolayer of the present inventionis generally formed by using, as the amphiphilic species, an alkylsilaneof general formula:Z₃C—(CF₂)_(n)—(CH₂)_(m)—SiX₃,wherein Z is H or F, n and m are independently any number from 0 to 21,for example any number from 2 to 12, with the proviso that the sum of nand m is at least 5 and for example at least 7, and X can be a halidesuch as Cl or Br, a C₁₋₄ alkoxy such as OMe or OEt, a hydroxy group orany other group which can be used to facilitate the binding of thesilicon to the surface of the optical element. The three X groups in asingle alkylsilane moiety may be the same or different. For example, thethree X groups in a single alkylsilane moiety are the same.

The alkylsilanes for use in the present invention may have linear alkylchains, however, the use of a branched chain alkylsilane is notexcluded, provided that reaction of the branched chain alkylsilane withthe surface of the optical element results in the formation of a selfassembled monolayer.

The alkylsilanes may have an aliphatic alkyl chain and may have afluoro-alkyl or perfluoro-alkyl chain. In each case the alkyl chain hasfrom 6 to 22 carbons, for example from 6 to 18 carbons or from 8 to 12carbons.

Specific examples of alkylsilanes which may be used to form theself-assembled monolayers of the present invention are CH₃(CH₂)₉SiX₃,CH₃(CH₂)₁₁SiX₃, CH₃(CH₂)₁₅SiX₃, CH₃(CH₂)₁₇SiX₃, CH₃(CH₂)₂₁ SiX₃,CF₃(CF₂)₅(CH₂)₂SiX₃, CF₃(CF₂)₇(CH₂)₂SiX₃ or CF₃(CF₂)₉(CH₂)₂SiX₃, whereinX is as defined in the general formula.

The X group bound to the silicon may be any of the substituentsmentioned in relation to the general formula, however, care must betaken when X is chlorine, since the by-products of the reaction mayroughen the original surface and cause unwanted effects such asscattering.

The self-assembled monolayers of the present invention may be formedfrom a single alkylsilane or from a mixture of alkylsilanes.

The application of the monolayer to the surface of the optical elementgenerally takes place after the surface has been cleaned. Methods ofcleaning are well known in the art and generally include treatment usingUV-ozone or plasma. This cleaning removes any contaminants which mayaffect the modification of the surface of the optical element. Thecleaning will leave a naturally adsorbed layer of water at the surface,such a layer assists in the production of a high quality monolayer.

Once cleaned, the surface of the optical element can be reacted with themodifying agent. This reaction is generally performed by reaction eitherin the gaseous phase or as an immersion reaction, however other methodsof applying a monolayer, such as spin coating, may also be used. Ifperformed in the gaseous phase, the cleaned element is placed in avessel with some modifying agent, the vessel is evacuated to a pressureless than about 1 mbar and the system is allowed to stand. The reactiontime is dependent upon the nature of the surface, the modifying agentand the amount of water which is present on the surface, however, itgenerally takes 6 to 24 hours. The reaction time can be reduced, forexample by raising the temperature, but the formation of localized threedimensional structures should be avoided. If performed by way of animmersion reaction, the cleaned substrate is placed into a solution ofthe modifying agent. The solvent of the solution is dependent upon thenature of the modifying agent but, in the case of alkylsilanes, suitablesolvents include alkanes, such as heptane or octane, aromatics, such astoluene, and alcohols, such as ethanol. Furthermore, when usingalkylsilanes the presence of water in the solution should be avoided.The reaction time for an immersion reaction again depends on the natureof the surface and modifying agent but is generally from 10 minutes to 2hours. When forming a self-assembled monolayer on the surface of anMo/Si multilayer mirror, the temperature of reaction must be kept lowsince interlayer diffusion can commence above 100° C.

After the monolayer has been applied, the modified element can bestabilized. The time and temperature needed for the stabilization isdependent upon the nature of the modifying agent, however, aging in airfor a few days or by baking at about 50° C. for a few hours is generallysufficient.

The surface coverage of the modification agent on the surface of theoptical element has to be sufficient such that a self-assembledmonolayer is formed. Typically, the surface coverage of the opticalelement by the self-assembled monolayer is around 70%, for example thecoverage is around 80%, for example the coverage is around 90%.

The monolayer should be thick enough to be stable under the irradiationconditions of the lithographic apparatus, however, it should not be toothick that it absorbs too much radiation. The thickness should be in therange of 0.5 to 10 nm, for example from 1 to 5 nm and for example from 1to 2.5 nm.

The two main pathways of reflection reduction of an optical element in alithographic apparatus are ascribed to oxidation of the surface of theelement due to the presence of water, and carbon build up on the surfaceof the element due to the presence of hydrocarbons in the system. Thepresent invention acts to decrease the effects of reflection reductionby both of these pathways.

Firstly, the formation of a self-assembled monolayer on the surface ofthe optical element acts to change the surface of the optical elementfrom being hydrophilic in character to being hydrophobic in character.Thus, the presence of water at the surface is reduced and the effects ofoxidation consequently lessened. Secondly, the use of an aliphatic orfluoroalkyl modifying agent results in the formation of a surface whichis non-polar and of low energy. Consequently, a large variety ofhydrocarbon compounds are unable to stick to the modified surface in away that would be possible if the surface was unmodified. Furthermore,the layer itself does not act to significantly reduce the reflectivityof the optical element and has also been shown to be stable under theconditions of use.

The self-assembled monolayers of the present invention have been foundto be stable after 50 hour exposure to an e-gun, i.e. there was found tobe no loss in the reflectivity of a multilayer mirror. After 100 hrunder such exposure there was found to be a loss of reflectivity, thisloss being in the region of 9%. However, such conditions are muchharsher than those experienced during the use of a lithographicprojection apparatus.

It has been found that the self-assembled monolayers of the presentinvention can be applied or reapplied in situ to the surface of anoptical element of a lithographic projection apparatus. Such a procedurecan be performed by releasing a modifying agent into the vacuum chamberwhich contains the optical elements. The procedure can thus be performedwithout disassembly of the apparatus, however, is not performed duringexposure to EUV radiation. Such a procedure represents a significantadvantage since it avoids the necessity of removing the mirrors from thesystem, thus the procedure can be performed both quickly and without therisk of exposing the mirrors to contamination.

According to a further aspect of the invention there is provided anoptical element as discussed above.

According to an even further embodiment of the present invention, adevice manufacturing method includes providing a beam of radiation;patterning the beam of radiation; and projecting the patterned beam ofradiation onto a target portion of a substrate, wherein at least oneoptical element on which the beam of radiation is incident has, on itssurface, a hydrophobic self-assembled monolayer.

Although specific reference may be made in this text to the use of theapparatus according to the invention in the manufacture of IC's, itshould be explicitly understood that such an apparatus has many otherpossible applications. For example, it may be employed in themanufacture of integrated optical systems, guidance and detectionpatterns for magnetic domain memories, liquid-crystal display panels,thin-film magnetic heads, etc. One of ordinary skill in the art willappreciate that, in the context of such alternative applications, anyuse of the terms “reticle”, “wafer” or “die” in this text should beconsidered as being replaced by the more general terms “mask”,“substrate” and “target portion”, respectively.

In the present document, the terms “radiation” and “beam” are used toencompass all types of electromagnetic radiation, including ultravioletradiation (e.g. with a wavelength of 365, 248, 193, 157 or 126 nm) andEUV (extreme ultra-violet radiation, e.g. having a wavelength in therange 5-20 nm), as well as particle beams, such as ion beams or electronbeams.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described, by way of exampleonly, with reference to the accompanying schematic drawings in which:

FIG. 1 depicts a lithographic projection apparatus according to anembodiment of the invention; and

FIG. 2 depicts an optical element including a self-assembled monolayeraccording to the present invention.

In the Figures, corresponding reference symbols indicate correspondingparts.

DETAILED DESCRIPTION

FIG. 1 schematically depicts a lithographic projection apparatus 1according to an embodiment of the invention. The apparatus 1 includes abase plate BP; a radiation system Ex, IL configured to supply a beam ofradiation PB of radiation (e.g. EUV radiation), which in this particularcase also includes a radiation source LA; a first object (mask) table MTprovided with a mask holder configured to hold a mask MA (e.g. areticle), and connected to a first positioning device PM that accuratelypositions the mask with respect to a projection system or lens PL; asecond object (substrate) table WT provided with a substrate holderconfigured to hold a substrate W (e.g. a resist-coated silicon wafer),and connected to a second positioning device PW that accuratelypositions the substrate with respect to the projection system PL. Theprojection system or lens PL (e.g. a mirror group) is configured toimage an irradiated portion of the mask MA onto a target portion C (e.g.including one or more dies) of the substrate W.

As here depicted, the apparatus is of a reflective type (i.e. has areflective mask). However, in general, it may also be of a transmissivetype, for example with a transmissive mask. Alternatively, the apparatusmay employ another kind of patterning device, such as a programmablemirror array of a type as referred to above.

The source LA (e.g. a discharge or laser-produced plasma source)produces a beam of radiation. This beam is fed into an illuminationsystem (illuminator) IL, either directly or after having traversed aconditioning device, such as a beam expander Ex, for example. Theilluminator IL may include an adjusting device AM that sets the outerand/or inner radial extent (commonly referred to as σ-outer and σ-inner,respectively) of the intensity distribution in the beam. In addition, itwill generally include various other components, such as an integratorIN and a condenser CO. In this way, the beam PB impinging on the mask MAhas a desired uniformity and intensity distribution in itscross-section.

It should be noted with regard to FIG. 1 that the source LA may bewithin the housing of the lithographic projection apparatus, as is oftenthe case when the source LA is a mercury lamp, for example, but that itmay also be remote from the lithographic projection apparatus, theradiation beam which it produces being led into the apparatus (e.g. withthe aid of suitable directing mirrors). This latter scenario is oftenthe case when the source LA is an excimer laser. The present inventionencompasses both of these scenarios.

The beam PB subsequently intercepts the mask MA, which is held on a masktable MT. Having traversed the mask MA, the beam PB passes through thelens PL, which focuses the beam PB onto a target portion C of thesubstrate W. With the aid of the second positioning device PW andinterferometer IF, the substrate table WT can be moved accurately, e.g.so as to position different target portions C in the path of the beamPB. Similarly, the first positioning device PM can be used to accuratelyposition the mask MA with respect to the path of the beam PB, e.g. aftermechanical retrieval of the mask MA from a mask library, or during ascan. In general, movement of the object tables MT, WT will be realizedwith the aid of a long-stroke module (coarse positioning) and ashort-stroke module (fine positioning), which are not explicitlydepicted in FIG. 1. However, in the case of a wafer stepper (as opposedto a step and scan apparatus) the mask table MT may just be connected toa short stroke actuator, or may be fixed. The mask MA and the substrateW may be aligned using mask alignment marks M₁, M₂ and substratealignment marks P_(I), P₂.

The depicted apparatus can be used in two different modes:

-   1. In step mode, the mask table MT is kept essentially stationary,    and an entire mask image is projected at once, i.e. a single    “flash,” onto a target portion C. The substrate table WT is then    shifted in the X and/or Y directions so that a different target    portion C can be irradiated by the beam PB;-   2. In scan mode, essentially the same scenario applies, except that    a given target portion C is not exposed in a single “flash.”    Instead, the mask table MT is movable in a given direction (the    so-called “scan direction”, e.g., the Y direction) with a speed v,    so that the beam of radiation PB is caused to scan over a mask    image. Concurrently, the substrate table WT is simultaneously moved    in the same or opposite direction at a speed V=Mv, in which M is the    magnification of the lens PL (typically, M=¼ or ⅕). In this manner,    a relatively large target portion C can be exposed, without having    to compromise on resolution.

FIG. 2 schematically depicts an optical element, in this case areflector, in a projection system according to the present invention.The optical element includes a self-assembled monolayer 2 (e.g. analkylsilane self-assembled monolayer); an Mo/Si multilayer mirror stack3; and a substrate 4, which supports the multilayer.

The surface of an Mo/Si multilayer mirror was prepared by cleaning usingUV/ozone or an oxygen plasma reactor. In such a process a thin oxidefilm usually remains at the surface and the presence of such ahydrophilic surface results in a thin layer of water molecules beingretained at the surface. Such water molecules are desirable for theproduction of a high-quality SAM film.

The mirror was placed in a pre-vacuum chamber (˜10⁻¹ mbar) together withthe alkylsilane, in this case perfluorodecyltriethoxysilane. Generally,approximately 200 μl of alkylsilane is sufficient to coat 0.1 m² of themirror surface. The reactants were then left for approximately 12 hourssuch that a self-assembled monolayer formed on the surface of themirror.

The monolayer was stabilized by either aging in air for a few days or bybaking at 50° C. for a few hours. The modified surface was found to havea film thickness of approximately 1.2 nm. Measurement of the watercontact angle of the modified surface gave a result of ˜110°,corresponding to a surface energy of about 10-20 mJ/m².

Comparative Example 1 is a standard Mo/Si multilayer mirror. The surfaceof this mirror exhibits a water contact angle of 5-15°. The reflectivity(R) of such a mirror is initially 68%, however, this reflectivitydecreases in a linear fashion during the conditions of use in an EUVlithography apparatus such that the reflectivity loss ΔR/R isapproximately 8% in a twenty hour period.

Example 1 was first exposed for 24 hours in 10⁻⁶ mbar H₂O and then for24 hours in 10⁻⁸ mbar diethylphthalate. After these treatments, thewater contact angle of the surface of the mirror was measured and therewas found to be no significant change when compared to that measuredpreviously. The self-assembled monolayer was also found to be stable attemperatures up to 400° C. in both air and N₂.

The reflectivity of Example 1 was found to be 67%. The reflectivity wasalso measured after exposure for 50 hours to an e-gun, replicating theeffects of exposure to EUV radiation, and the reflectivity afterexposure was found to be essentially the same as that prior to exposure.However, after 100 hours there was a reflectivity loss ΔR/R ofapproximately 9%.

While specific embodiments of the invention have been described above,it will be appreciated that the invention may be practiced otherwisethan as described. The description is not intended to limit theinvention.

1. A lithographic projection apparatus, comprising: a radiation systemconfigured to provide a beam of EUV radiation; a support configured tosupport a patterning device, the patterning device configured to patternthe beam of radiation according to a desired pattern; a substrate tableconfigured to hold a substrate; and a projection system configured toproject the patterned beam onto a target portion of the substrate,wherein the radiation system and/or the projection system contains anoptical element comprising a substrate; a Mo/Si multilayer stack on thesubstrate; and a hydrophobic self-assembled monolayer on the multilayerstack.
 2. An apparatus according to claim 1, wherein the optical elementhas a peak reflectivity for radiation having a wavelength of about 5 nmto about 50 nm.
 3. An apparatus according to claim 2, wherein thereflectivity of the optical element is greater than about 40%.
 4. Anapparatus according to claim 1, wherein the optical element is anear-normal incidence mirror, a grazing incidence mirror, an integrator,or a sensor.
 5. An apparatus according to claim 4, wherein thenear-normal incidence mirror is a Mo/Si multilayer mirror.
 6. Anapparatus according to claim 4, wherein the integrator is a scatteringplate.
 7. An apparatus according to claim 4, wherein the sensor is animage sensor or a spot sensor.
 8. An apparatus according to claim 1,wherein the self-assembled monolayer covers about 70% of a themultilayer stack.
 9. An optical element, comprising: a substrate; aMo/Si multilayer stack on the substrate; and a hydrophobicself-assembled monolayer on the multilayer stack.
 10. An optical elementaccording to claim 9, wherein the optical element has a peakreflectivity for radiation with a wavelength of about 5 nm to about 50nm.
 11. An optical element according to claim 9, wherein thereflectivity of the optical element is greater than about 40%.
 12. Anoptical element according to claim 9, wherein the optical element is anear-normal incidence mirror, a grazing incidence mirror, an integrator,or a sensor.
 13. An optical element according to claim 12, wherein thenear-normal incidence mirror is a Mo/Si multilayer mirror.
 14. Anoptical element according to claim 12, wherein the integrator is ascattering plate.
 15. An optical element according to claim 12, whereinthe sensor is an image sensor or a spot sensor.
 16. An optical elementaccording to claim 9, wherein the self-assembled monolayer covers about70% of the multilayer stack.
 17. A device manufacturing method,comprising: providing a beam of radiation; patterning the beam ofradiation; and projecting the patterned beam of radiation onto a targetportion of a substrate, wherein at least one optical element on whichthe beam of radiation is incident has, on its surface, a hydrophobicself-assembled monolayer.
 18. A method according to claim 17, whereinthe optical element has a peak reflectivity for radiation having awavelength of about 5 nm to about 50 mm.
 19. A method according to claim17, wherein the reflectivity of the optical element is greater thanabout 40%.
 20. A method according to claim 17, wherein theself-assembled monolayer covers about 70% of a surface of the opticalelement.